Shebang should refer to python, not python3

This is better for compatibility with pyvenvs or virtualenvs.
Make __doc__ string more readable
Renaming containerPlugins.py → container_plugins.py
Make data store configurable via --plugin

* Moving dict like container data structure plugins into subfolder plugins
* Adding argparse CLI param --plugin to specify data store backend
  This can be now be something like
  redis://username:password@localhost:6379/0
  or
  filedict://db.shorturls.sqlite3